2. Field of the Invention
The present invention generally relates to a method and apparatus for generating underwater sharp, impulsive acoustic signals, which are especially useful in marine seismic exploration and, in particular, to such acoustic sources which periodically and abruptly create a gas bubble at a sufficient depth below the water surface to allow the bubble to expand and contract.
2. Description of the Prior Art
Certain seismic sources such as explosives, airguns, gas exploders, etc., are purposely fired deep under water. It is well known that such firing creates a gas bubble or cavity and that the water acquires oscillatory energy which generates acoustic pressure wavelets, each consisting of a desired "primary" acoustic pressure pulse P.sub.o, which is especially useful for most seismic exploration work, and which is followed by an oscillating succession of undesired "secondary" (sometimes called "bubble") acoustic pulses of decreasing amplitude. In this specification, the words "bubble" and "cavity" will be used interchangeably.
For example, an airgun explosively releases a high pressure gas bubble into the water which creates the desired primary pulse having a maximum amplitude P.sub.o. After the released high-pressure gas bubble impulsively contacts the surrounding water, it continues to expand as the water first accelerates outwardly and later decelerates until the cavity attains a maximum diameter and comes to rest, at which time the pressure within the cavity is much less than the surrounding hydrostatic pressure.
When the expanding bubble reaches its maximum diameter there is practically a vacuum inside the bubble, the kinetic energy of the surrounding water is zero, and this water possesses maximum oscillatory potential energy which, if not suppressed, will change into kinetic energy, back into potential energy, etc., for a duration of several cycles, each having an oscillatory time period T.
The water gains maximum potential energy at 1/2 T, at which time the water is ready to change course and rush inwardly to implode the gas in the bubble. After one complete cycle, i.e., at time T, the bubble is recompressed into a relatively small-diameter, high-pressure bubble. The surrounding water comes to an abrupt stop resulting in a first positive acoustic secondary pulse P.sub.2 which is mainly dependent upon the maximum kinetic energy acquired by the inwardly-moving water. The less kinetic energy acquired by the water, the smaller P.sub.2 will be.
Thus, the secondary pulse problem starts when the surrounding water for the first time violently implodes the bubble to a minimum diameter. When the bubble recompresses and attains a minimum diameter or volume, the kinetic energy is again zero and the potential energy is mainly contained within the recompressed gas inside the bubble. This potential energy causes the bubble to again explode in its oscillatory scheme. In this manner, the oscillatory energy stored in the water produces several successive secondary pulses of deceasing amplitude until a portion of the energy of oscillation becomes dissipated by natural processes, such as turbulence, and the remaining portion is consumed to produce the undesired secondary pulses.
The number of such bubble explosions (expansions) and implosions (contractions) may vary, but typically four to six significant secondary pulses can be expected after each primary pulse P.sub.o which is generated by the seismic source.
Hence, a substantial portion of the acoustic energy released by the seismic source goes to waste. This is so because only a portion of the energy contained in the released gas is used to produce the desired primary seismic pulse P.sub.o, while the remaining and substantial portion of the energy becomes converted into harmful secondary seismic pulses which must be suppressed.
Such secondary pulses are now being attenuated by using large arrays of differently sized airguns. All the airguns in the array are fired simultaneously so that the P.sub.o pulses are in phase for addition. Most of the secondary pulses are out of phase and therefore they cancel each other out.
In seismic exploration, both the primary and secondary acoustic pulses act as distinct acoustic disturbances which travel in the water in all directions, penetrate the earth, strike one or more rock formations or reflectors, and then return into the body of water.
The primary and secondary pulses produce reflected seismic wavelets. But, since the secondary and their reflected waves occur at times when the reflected primary waves also return from the subterranean reflectors, it will be apparent that the secondary pulses and their reflected wavelets interfere with the reflected primary waves.
Because the reflected and secondary waves and the reflected primary waves are similar in shape, no practical way has yet been found for distinguishing between them.
The known prior art found neither a practical nor an economical solution for dealing with the "bubble" or secondary pulse problem when using a single explosive-type seismic source. For this reason, and as previously mentioned, marine acoustic sources are used now mostly in arrays to achieve bubble cancellation.
In conducting marine seismic surveys, the reflected primary and secondary seismic waves are sensed by detectors within a towed streamer cable. The detectors faithfully transform the received and acoustic seismic waves into corresponding electric signals which are processed into seismic traces that contain appreciable noise. This noise is due mostly to the oscillatory secondary pulses which accompany each primary pulse.
Under these noisy conditions, computations of the depths at which the rock formations lie become very difficult and sometimes altogether impossible. The noise hinders the main object of the seismic exploration, which is, of course, to identify the various sub-bottom formations from an interpretation of the seismogram sections produced by the seismic survey.
The secondary-to-primary ratio P.sub.2 /P.sub.o is the yardstick by which all marine seismic sources are measured as to bubble suppression. An "ideal" source is said to be that source which has a ratio P.sub.2 /P.sub.o =0 for a frequency range from 0-125 Hz. The extent to which a particular seismic source approaches the ideal seismic source can be readily measured by measuring its P.sub.2 /P.sub.o ratio.
An ideal seismic source produces a single, short, sharp acoustic impulse having sufficient energy and no secondary pulses. Sharp impulses are needed to improve the definition of seismic reflections, because resolution is inversely proportional to the time-width of the impulse: the larger the time-width of the impulse, the less desirable it is.
Fired near the water surface, a dynamite charge closely approximates the ideal seismic source, because the bubbles resulting from each explosion are vented immediately into the atmosphere, hence there are no bubble implosions.
If not fired near the water surface, explosive seismic sources will produce undesired secondary pulses, unless some form of implosion suppression is utilized. Explosive seismic sources include explosives, airguns, gasguns, expandable sleeve devices in which propane and oxygen are mixed to cause internal combustion, etc. All of these share the common bubble problem for which there has been no fully satisfactory solution, even though there has been a long-felt need to find a mechanism to enhance the desired primary pulse at the expense of the undesired secondary pulses.
In the absence of such a mechanism, many attempts have been made in the past twenty-five years or more by the oil companies and their seismic contractors to develop techniques for reducing the burden--both financial and technological--imposed by the generation of the undesirable secondary pulses. These efforts were directed toward attenuating the oscillatory secondary pulses and/or to reduce their ill effects.
From the very early introduction of marine seismic sources, there was a continuous need for effective and economical bubble suppression devices. That need and the various solutions offered to fill that need are well described in the technical and patent literature. Only a few of these will be discussed below to illustrate the severity of the bubble problem and the diversity of attempts to solve it.
One early mechanical technique attempts to prevent the secondary pulses from traveling vertically downward towards the water bottom by substantially surrounding the gas bubble source with a container or cage having perforations, so that the expanding gas bubble would have to do work in order to force water through the perforations. The work done by the expanding gas bubble dissipates its internal energy, so that the ensuing secondary pulses will have reduced amplitudes. This technique was used in a seismic source trademarked FLEXOTIR.
A serious limitation inherent in this technique is that the desired primary pulses also become reduced in strength because they can travel freely only through the available perforations. Also, the perforated cage becomes subjected to rapid deterioration, due to the great stresses to which it becomes subjected when large differential pressures become exerted across its wall.
Various software programs have been also developed, for example, in connection with the MAXIPULSE (trademark) seismic source, which utilize fast and powerful digital computers that produce seismograms from which the detected noisy seismic waves caused by the deleterious bubble effects have been removed so that the seismograms can be easier interpreted by the geophysicists. However, running such programs requires the use of expensive computer time and manpower, see U.S. Pat. No. 3,592,286.
Other techniques are based on air being injected into the expanding bubble for shaping the secondary pulses. The known applications of the air injection technique have led prior art workers to very disappointing results and most of them were abandoned.
In U.S. Pat. No. 3,371,740, the injection of air during the expansion of the cavity might increase the size of the cavity without reducing the amount of kinetic energy stored in the water. The injected atmospheric pressure is too low. The cavity is allowed to implode for too long at a time, and therefore the water is allowed to acquire too much kinetic energy. For a firing depth of 30 ft, it can be shown that P.sub.2 /P.sub.o approaches 40%, which is generally unacceptable.
In U.S. Pat. No. 3,454,127, the injection is started too early during the expansion of the bubble. The flow rate of the injection is subsonic, which is insufficient to establish hydrostatic pressure inside the cavity within the required time interval, unless an impractically large gas volume is utilized. Also, the volatilization of a material cannot be used because a material cannot volatilize within the few milliseconds available for achieving hydrostatic pressure inside the cavity.
In U.S. Pat. No. 3,601,216, the final pressure established inside the cavity is not hydrostatic. The volume of gas injected is too small: only 9 ft.sup.3 instead of 60 ft.sup.3, which is needed. The start of the air injection is not defined, or at most it is defined as "when" the bubble is expanding, which is very imprecise. The time interval allowed for the injection is much too long, "preferably 50 ms," instead of the maximum 20 ms allowed. It uses low injection pressure (150 psi) which leads to severe practical and technical difficulties.
U.S. Pat. No. 3,653,460, involves the use of an airgun having a secondary chamber in addition to a main chamber. Upon the release of the compressed air from the main chamber, the secondary chamber releases its air into the main chamber, and thence out through the discharge ports in the airgun and into the expanding bubble. Air from the secondary chamber is throttled across an orifice which is contained in a dividing wall between the chambers. As a result, the flow rate is maximum when the bubble is small and expanding, and the flow rate is reduced when the bubble reaches its maximum size. This reduction in flow is due to the pressure decrease in the secondary chamber during the bubble expansion. In column 5, lines 1-30 of the last mentioned patent, it is shown that the volume of air injection required to bring the P.sub.2 /P.sub.o ration down to 14% is 6.4 times the volume needed to generate the main pulse, which is inefficient, wasteful, and very costly in money and energy consumption. While the quantity of air being injected into the bubble may have some effect on the P.sub.2 /P.sub.o ratio, it is generally not enough to keep this ratio within acceptable limits which are already achieved when using tuned airgun arrays, and does not allow the source to become a point source.
Due to the inefficiency or impracticability of known bubble suppression techniques, the seismic industry was obliged to employ a "tuned" array of seismic sources. Typically, these sources are airguns of markedly different sizes.
When all the airguns in a tuned array (using airguns of different sizes) are fired simultaneously, theoretically the amplitude of the resulting primary pulse of the array will be equal to the sum of the amplitudes of the individual primary pulses generated by the individual acoustic sources in the array, whereas the amplitudes of the secondary pulses will be reduced because (1) they are not in phase, (2) they occur at different items, and (3) they have random frequencies.
Nevertheless, the present state of the art in suppressing bubble pulses has been achieved by these "tuned" airgun arrays, which are now widely used.
Even though the array technique is now the standard in this art, it still has serious drawbacks because it is only a composite of individual sources, each source lacking a narrow, sharp acoustic pulse as required. Also, it is very expensive to build such an array because it requires a large number of differently-sizes airguns, as well as heavy and expensive air compressors to provide the volume of 2000 psi air consumed by the large number of airguns. The breakdown of any gun in the array damages the resultant signature of the array, leading to frequency distortion.
There is also a need to maintain on the seismic boat a large inventory of spare parts to keep the differently-sized sources operational. The spare part problem is very serious, because in many parts of the world they are not available and they must be flown in from the home base. Many parts break down daily and some weekly due to salt water, pollution, unsuspected debris, high pressure, etc.
As a consequence, the art has been obliged to use techniques which are known to have serious drawbacks in order to accommodate the demand for marine seismic prospecting. Most importantly, the prior art has failed to suppress the secondary pulses to acceptable levels which would make it possible to utilize a single-point seismic source, or an array of such sources of equal size.
In general, the known bubble suppression devices have the following drawbacks, taken singularly or in combination: they are cumbersome; they require excessive support machinery and space therefor; they require very large compressors and excessive energy to run them; they are expensive to maintain; and they require an excessive inventory of expensive replacement parts to maintain in operational condition the different sizes of seismic sources now utilized to form arrays.
Accordingly, it is a broad object of this invention to remedy the above and other known shortcomings of the prior art, and to provide a practical, economical, and fully effective method for total bubble suppression.
It is another object of this invention to provide a seismic source which is bubble-free and therefore can be used alone for seismic prospecting. If more power is needed, my sources can be used to construct an array which is very beneficial, as will be subsequently described.
It is yet a further object to provide a bubble-free seismic source which is characterized by having a substantially flat power spectrum over a relatively broad frequency range, and which produces a single sharp acoustic impulse followed by negligible secondary pulses.
It is another object of this invention to provide such a novel energy source which has effective bubble suppression means, which is energy-efficient, which lends itself to become easily incorporated into existing marine seismic energy sources and methods of using them without affecting the utility of such known sources and methods, which is simple in design, compact in use, and relatively inexpensive to manufacture and maintain, and in which the volume of air required for aborting the implosion is reduced to a small fraction of what was generally believed to be necessary to achieve adequate bubble suppression.